“Seismic” waves are mechanical perturbations that travel in the Earth at a speed governed by the acoustic impedance of the medium in which they are travelling. The acoustic (or seismic) impedance, Z, is defined by the equation:Z=Vρ
where V is the seismic wave velocity and ρ (Greek rho) is the density of the rock.
The acoustic pulses are typically generated by vibrating the earth with specially equipped trucks, a technology known as “Vibroseis” that was invented by Conoco about 50 years ago. When a seismic wave travelling through the ground encounters an interface between two materials with different acoustic impedances, some of the wave energy will reflect off the interface and some will refract through the interface.
At its most basic, the seismic reflection techniques consist of generating seismic waves and measuring the time taken for the waves to travel from the source, reflect off an interface and are detected by an array of receivers (or geophones) at the surface. Knowing the travel times from the source to various receivers, and the velocity of the seismic waves, a geophysicist then attempts to reconstruct the pathways of the waves in order to build up an image of the subsurface.
Oil and gas companies rely on 3D seismic data to better delineate fields and identify new reserves, but most companies are now asking more of their 3D seismic surveys. Advances in acquisition, processing and interpretation techniques are being used for complete volume coverage of the reservoir. High-resolution borehole seismic surveys help combine the surface seismic with log and core data to allow log properties such as lithology, porosity and fluid type to be mapped field-wide. With this more complete understanding of the reservoir, production engineers can optimize development and recover additional reserves.
In the process of acquiring seismic data, seismic energy is generally applied over time where the vibrators begin a sweep by vibrating initially at a low frequency and progressively increasing the frequency such that an entire sweep of the frequency range is delivered within a certain time period. Sweeps of four to eight seconds have been standard practice for years, but longer sweeps are becoming increasingly common with sixteen second sweeps and forty eight second sweeps now being used.
The costs for a seismic survey can be quite expensive and therefore much effort has gone into improving the efficiency of seismic surveying—getting the most from a particular dataset. One advance is to operate several seismic vibrators at the same time all making a similar sweep, but in different phases with respect to one another. In other words, if the baseplate of one vibrator is going up while another is going down, the two vibrators would be about 180 degrees out of phase. Operating four vibes that are out of phase with respect to one another is known and commercially in use as the HFVS or ZenSeis® geophysical prospecting systems, among others.
Typically, with four vibrators, at least four separate sweeps are performed where the phase relationship between the vibrators is changed between sweeps to enhance the distinctiveness of each vibrator in the data record. The distinctiveness of the data sets, allows the data to be separated and accorded to a single source, thus providing the most information in a given amount of time.
Being out of phase at an orthogonal relationship to one another is not the most effective way of distinguishing the sweep data. If one vibrator is at zero degrees phase and the next vibrators is 90 degrees ahead, the next is 180 degrees ahead and the last is 270 degrees ahead, this combination is described as orthogonal, such that everything is 90 or 180 degrees different from one another. In this arrangement, echoes and in particular even harmonics from the subsurface geological structures are created and are somewhat difficult to distinguish from the principal reflections. Thus, the data is easily confused, and the data from each of the vibrational sources may not be accurately separated.
Thus, it is preferred that phase differences or offsets are non-orthogonal, which means the phase differences between the four vibrators is not equal to 90 degrees or similar angles. The non-orthogonal phase differences have to be optimally chosen to minimize leakages of the harmonics and cross talk between the sources during separation. U.S. Pat. No. 7,295,490 explains the derivation of the phase encoding in more detail. Unfortunately, equipment and circumstances are never perfect and vibrators that are supposed to be out of phase may actually operate at a phase offset that becomes difficult to distinguish due to equipment drift or wear and tear. In this circumstance, it may not be apparent to the operators that the phase excursion or drift has happened until after much or all of the survey is completed. The cost of re-running the survey or the portions of the survey may not be justifiable.
Many efforts have been made to address phase issues and to obtain the most information from the seismic data.
U.S. Pat. No. 8,467,267, for example, describes an approach whereby the seismic recording system comprises a) two or more seismic energy sources, and b) one or more data recorders, wherein said seismic energy sources are operated asynchronously with a random or non-uniform lag between consecutive sweeps. The energy source signatures are recorded, and said data is synchronized through inversion of the recorded data by the energy source signatures.
US20100208554 relates to methods and equipment for acquiring and processing marine seismic data that correct source movement during inversion. By correcting source movement during inversion, multiple data sets may be acquired independently during overlapping time periods thus reducing the number of sweeps required, generating greater amounts of data, and simplifying data processing. In more detail, the system uses two or more independent phase encoded sources to transmit multiple simultaneous sources, and said independent phase encoded source receiver signals are separated and stacked during inversion of the recorded seismic signals.
US20120033529, also by the inventors, attempts to solve the phase error problem by a technique that quickly creates a more accurate source signature delivered by analysis of the source generated data contamination present in the separated data where such data contamination is the presence of one source's energy in another source's data after separation. The technique is to invert a segment of the data using a seed source signature and compute an error that reflects the data contamination observed in the separated source data. The source signature is iteratively revised as the segment is continually inverted with the goal of finding the optimal source signature that provides the lowest computed error. The source signature that provides the lowest error is, or is very close to, the true source signature and is then used in the separation process for the entire composite data set.
US2012028775 attempts to address phase errors by correcting data prior to inversion where the correction is provided to correct for the filtering effect of the earth. The method includes the acquisition of seismic data to create a data set for a survey area. An initial source wavelet is identified and an expected response to the initial source wavelet by one or more seismic receivers taking into consideration the geometric relationship of the receivers to the source is identified. A computed earth response is created for each source and receiver pair based on the geology between the source and receiver and the computed earth response is applied to the expected response to the initial source wavelet to create a source true estimated wavelet for each source receiver pair. The source true estimated wavelet is then used for at least one further processing step of the acquired data set such as inverting, separating, de-signaturing or wavelet de-convolving.
US20120039150 relates to the acquisition of seismic data using many seismic sources simultaneously or where the sources are emitting in an overlapping time frame but where it is desired to separate the data traces into source separated data traces. The key is having each seismic source emit a distinctive pattern of seismic energy that may all be discerned in the shot records of all of the seismic receivers. Distinctive patterns are preferably based on time/frequency pattern that is distinctive like an easily recognized song, but may include other subtle, but recognizable features such a phase differences, ancillary noise emissions, and physical properties of the vibes such as the weight and shape of the pad and the reaction mass and the performance of the hydraulic system and prime energy source.
Inventors' own extensive testing data derived from vertical seismic profile (VSP) projects suggest that the vibes true phase and the reported phase by the vibe sweep controller has an overall bulk shift and a frequency dependent shift. It was also noted from other tests, that this shift may vary per the number of sweeps at a particular spot of ground and with the individual vibrator. Testing of the inverted results show that as little as a few degrees of error causes significant cross talk between the shots. To improve the separation between the vibes on different shot points this error must be compensated for.
Although many people have attempted to address known vibration phase error corrections through various feedback circuits, no one has published the use of independent measurements to quantify the errors themselves, especially acknowledging the frequency-dependent phase error. Thus, one of the sources of error is currently unrecognized and/or not compensated for.